name: tdd-kotlin
description: >
TDD patterns for Kotlin CA/FP projects using Kotest + MockK + Arrow-kt. Use when writing
tests, designing test cases, or following the red-green-refactor cycle. Triggers on: test
file creation, writing value object tests, use case tests with mocked repositories,
controller tests, property-based testing, coEvery/coVerify patterns.
TDD for Kotlin CA/FP
Test Stack
| Library |
Purpose |
kotlin("test") |
@Test annotation |
kotest-assertions-core |
shouldBe, shouldBeInstanceOf |
kotest-assertions-arrow |
shouldBeRight(), shouldBeLeft() |
kotest-property |
checkAll(Arb.xxx()) { } |
mockk |
mockk<T>(), coEvery, coVerify |
kotlinx-coroutines-test |
runTest { } |
Layer Test Patterns
| Layer |
Approach |
| Domain |
No mocks. checkAll for boundaries, shouldBeRight/shouldBeLeft for Either results |
| Application |
mockk<Repository>(), coEvery/coVerify, runTest {} |
| Presentation |
mockk<UseCase>(), test each HTTP status code via response.statusCode shouldBe ... |
| Infrastructure |
Integration tests only — requires test containers or H2 |
Key Assertions
result.shouldBeRight(expectedValue) // assert Right with specific value
result.shouldBeLeft().shouldBeInstanceOf<E>() // assert Left type
error.message shouldContain "expected text" // assert error message content
coVerify(exactly = 1) { repo.save(any()) } // verify call count
coVerify(exactly = 0) { repo.save(any()) } // verify NOT called
Test Naming
@Test
fun `of returns Left InvalidId when value is zero or negative`() = runTest { ... }
@Test
fun `execute returns Left FetchFailed when repository throws exception`() = runTest { ... }
For complete test skeletons per layer (domain/application/presentation): see references/test-patterns.md.
By